The conventional magnetic tunneling junction (MTJ) device is a form of ultra-high magnetoresistive device in which the relative orientation of the magnetic moments of parallel, vertically separated, upper and lower magnetized layers controls the flow of spin-polarized electrons tunneling through a very thin dielectric layer (the tunneling barrier layer) formed between those layers. When injected electrons pass through the upper layer they are spin polarized by interaction with the magnetic moment of that layer. The majority of the electrons emerge polarized in the direction of the magnetic moment of the upper layer, the minority being polarized opposite to that direction. The probability of such a polarized electron then tunneling through the intervening tunneling barrier layer into the lower layer then depends on the availability of states within the lower layer that the tunneling electron can occupy. This number, in turn, depends on the magnetization direction of the lower electrode. The tunneling probability is thereby spin dependent and the magnitude of the current (tunneling probability times number of electrons impinging on the barrier layer) depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer. The MTJ device can therefore be viewed as a kind of bi-state resistor, since different relative orientations (e.g. parallel and antiparallel) of the magnetic moments will change the magnitude of a current passing through the device. In a common type of device configuration (spin filter), one of the magnetic layers has its magnetic moment fixed in direction (the reference layer), while the other magnetic layer has its magnetic moment free to move (the free layer). The magnetic moment of the free layer is then made to switch its direction from being parallel to that of the reference layer, whereupon the tunneling current is large, to being antiparallel to the pinned layer, whereupon the tunneling current is small. Thus, the device is effectively a two-state, or bi-stable resistor. The switching of the free layer moment direction (writing) is accomplished by external magnetic fields that are the result of currents passing through conducting lines adjacent to the cell. Once the cell has been written upon, the circuitry must be able to detect whether the cell is in its high or low resistance state, which is called the “read” process.
The use of magnetic fields externally generated by current carrying lines to switch the magnetic moment directions becomes problematic as the size of the MRAM cells decreases and, along with their decrease, so must the width of the current carrying lines. The smaller width lines require greater currents to produce the necessary switching fields, greatly increasing power consumption.
For this reason, a new type of magnetic device, called a spin torque transfer (STT) device, described by Slonczewski, (U.S. Pat. No. 5,695,164) has been developed, that seems to eliminate some of the problems associated with the excessive power consumption necessitated by the need for external switching fields. The spin transfer device shares some of the operational features of the conventional MTJ cell (particularly the read process) described above, except that the switching of the free layer magnetic moment (the write process) is produced internally by passage of the spin polarized current itself. In this device, unpolarized conduction electrons passing through the reference layer, whose magnetic moment is oriented in a given direction, are preferentially polarized by their passage through that layer by a quantum mechanical exchange interaction with the polarized bound electrons in the layer. Such a polarization can occur to conduction electrons that reflect from the surface of the magnetized layer as well as to those that pass through it. The efficiency of such a polarization process depends upon the crystalline structure of the layer. When such a stream of polarized conduction electrons subsequently pass through the free magnetic layer, whose polarization direction is not fixed in space, the now polarized conduction electrons exert a torque on the bound electrons in the free layer which, if sufficient, can reverse the polarization of the bound electrons and, thereby, reverse the magnetic moment of the free layer. The complete physical explanation of such a torque-induced reversal is complicated and depends upon induction of spin precession and certain magnetic damping effects (Gilbert damping) within the magnetic layer. That explanation will not be given herein. The use of a current internal to the cell to cause the magnetic moment reversal requires much smaller currents than those required to produce an external magnetic field from adjacent current carrying lines to produce the moment switching.
The equilibrium magnetic moments of the free and reference layers (i.e. when the conduction current is not passing through them) can lie either in the deposition plane of the magnetic layers or perpendicularly to that plane. Perpendicular magnetization is accomplished by engineering a perpendicular magnetic anisotropy (PMA) into the layer stack, within either the bulk of the layer material or at the interfaces of the layers. The present disclosure is directed at this PMA configuration.
Referring first to FIG. 1, there is shown a highly schematic diagram indicating how a typical, single STT MTJ cell, in the PMA configuration, is connected within a circuit that would typically contain a multiplicity of such cells. The cell (500) is shown as a three-layer fabrication, having a free layer (100), a reference layer (200) and a tunneling barrier layer (300) positioned between them. In accord with their PMA configuration, the free layer (100) is shown with a double ended arrow (150), indicating the two perpendicular directions that its magnetic moment will take under the action of a passing current. The reference layer is shown with a single ended arrow (250) to indicate its fixed perpendicular magnetic moment.
The cell (500) is positioned between two current carrying lines, a bit line (50), and a complementary bit line (60). An accessing transistor (70) is shown in the connecting line (80) between the complementary bit line (60) and the reference layer (200) of the cell. The gate electrode (75) of the transistor is connected to another current carrying line (90), called the write line. When a current in the write line activates the gate electrode (75), the transistor allows a current to pass through the cell between the bit line (50) and the complementary bit line (60), typically causing a flip of the direction of the magnetic moment (150) of the free layer, which is termed a write operation.
In general, the reference layer (200) exerts a magnetic field (denoted a “stray field”) on the free layer (100). This field is a result of magnetic charges (i.e. the divergence of the magnetization vector) on the opposite surfaces of the reference layer. This field usually favors magnetic alignment of the magnetization vectors, (150) and (250) of the two layers. The orientation of the magnetic moment of the free layer (relative to that of the reference layer) corresponds to the storage of a bit of information. Whether this bit is a “0”, corresponding to alignment and minimum resistance, or a “1” corresponding to anti-alignment and a maximum resistance, can be determined by a measurement of the current passing through the cell. To guarantee that the bit, once written, remains stable until it is overwritten, it is desirable that the free layer magnetic moment should not be influenced by the magnetic field of the reference layer. Therefore, the reference layer should be designed to exert a minimal field on the free layer, with an “ideal” design producing a zero field at the position of the free layer.
As an approach to minimization of the field of the reference layer, Heim et al. (U.S. Pat. No. 5,465,185), teach a reference layer formed as a synthetic antiferromagnetic structure (SAF).
Referring to FIG. 2, there is shown a highly schematic illustration of an SAF reference layer. The SAF structure is a three layer structure: (AP2)/non-magnetic/(AP1), where AP2 (10) and AP1 (20) are ferromagnetic layers, coupled, at equilibrium, in a mutually anti-parallel configuration of their magnetic moments (arrows (15) and (25)) across a non-magnetic coupling layer (30) that is typically a thin layer of ruthenium (0.9 nm). The reduced net magnetic moment of the coupled pair lessens the magnitude of the stray field acting on the free layer.
Another mechanism of minimizing the reference layer field is taught by Xi (US Patent Publication No. 2010/0102406). In this approach, which has been denoted a “partial etch” design, as illustrated schematically in FIG. 3, the free layer (100) and barrier layer (300) are partially etched back so that the free layer specifically is substantially narrower than the reference layer (200). The fringing field (50) of the magnetization (55) of the reference layer (200), which is primarily due to magnetic charges creating a dipole configuration at the lateral edges of the reference layer, is substantially removed from the free layer, so that its effects on the free layer are minimized. When the width of the reference layer approaches “infinity” (i.e. is very much wider than the free layer), the magnetic charges on its top and bottom surfaces would effectively cancel and produce a stray field of essentially zero magnitude. When the width of the reference layer is large, but finite, as in FIG. 3, the stray field is not reduced to zero, but its sources are the unbalanced upper and lower surface charges at the opposite lateral edges of the layer (as shown in the figure), much like the fringing electric field produced at the edges of a parallel plate capacitor. However, the partial etch design still does not alleviate the problems of the reference layer fields because even the stray field at the edges of a 500 nm diameter “simple” (not composite) reference layer is appreciable, the magnitude of H being: H>25 Oe.